This invention relates generally to molds for forming glassware, such as glass bottles.
Most glassware such as bottles is formed by either a "blow and blow" method or a "press and blow" method. In the blow and blow method, molten glass is introduced into a mold cavity and the interior of the bottle is formed by introducing gas into the molten glass, which gas forces the molten glass against the mold walls to form a rough shape having a cavity. The rough shape is then transferred to a finishing mold where gas pressure is used to force the shape against the mold walls, which imparts final shape to the bottle. The press and blow method is similar, except that a mechanical plunger is used to impart the rough shape having a cavity in the first step of the process. The present invention involves molds for use in the first step of these processes, in which molten glass at a temperature typically in the range of 1050.degree. C. to about 1200.degree. C. contacts the mold.
Conventional split molds for producing glass containers have been made from a gray cast iron, which is a material having high thermal conductivity and moderate wear resistance at temperatures up to about 500.degree. C. However, this material lacks corrosion resistance and is prone to "glass sticking", a phenomenon where the article being molded sticks to the mold surfaces defining the mold cavity. To avoid "glass sticking", these mold surfaces are swabbed with oil or other suitable release material. However, swabbing can result in inclusions and micro cracks in the finished product due to contamination of the mold by the swabbed material, which may reduce the strength of the glass. Also, failure to apply swabbing material uniformly over the mold surfaces can cause temperature discontinuities and non-uniform cooling, which again can result in cracking of the glassware. Still further, the molds become dirty due to swabbing, so that regular cleaning is required.
U.S. Pat. No. 3,888,647 discloses glass container molds made of low alloy nickel-chromium steels. A low alloy nickel-chromium steel is a steel having a total alloying element content less than or equal to about 8%.
Attempts have been made to improve the wear resistance of glass container molds by hardfacing the molding surfaces with nickel-based wear resistant materials. A disadvantage of this approach, however, is that insulating properties of the wear-resistant layer slow the cooling of the mold surface.
There is a need, therefore, for a mold which is constructed to mold high-quality glassware without the need to swab the mold surfaces defining the mold cavity. There is also a need for a mold of increased wear and corrosion resistance, and for a mold which provides more uniform cooling, increased productivity and increased reliability.